1. Field of the Invention
The present invention relates to a directional antenna control device, a beam selecting method therefor, and a program, and more particularly, to a method for controlling directivities of a plurality of array antenna elements provided at a base station that receive incoming radio waves.
2. Description of the Related Art
Great expectations have been focused on a CDMA (Code Division Multiple Access) system that will be a radio access system for next-generation mobile communication cellular system because it may significantly increase a subscriber capacity.
Such a CDMA system is, however, prone to interference that is produced on a base station receiving side due to signals from other users making an access on the same carrier at the same time and also produced on a mobile station receiving side due to signals transmitted from the base station to other users. To eliminate this interference, there has been provided array antenna-based technology (e.g., see “W-CDMA Mobile Communication System” (published by MARUZEN CO., LTD. on 25 Jun. 2001, edited by Keiji Tachikawa, Pages 79 to 86)).
The array antenna receiving signals by a plurality of antenna elements contributes to suppression of interference with signals of other users by applying complex weights to the received signals and combining the resulting signals to control amplitudes and phases of the received signals from each antenna so as thereby to form a directional beam. A multibeam system is one example of control methods for such an array antenna. FIG. 4 shows a block diagram showing a conventional directivity control device employing the multibeam system.
According to the multibeam system in FIG. 4, a receiving array antenna unit 1 receives signals by N antenna elements 11 to 1N (N is an integer grater than one) arranged close to each other, and then an A/D (Analog/Digital) conversion unit 2 converts the received signals from analog to digital at A/D converters 21 to 2N provided for the antenna elements 11 to 1N, respectively.
The received signals are multiplied by weighting factors calculated in advance, in a reception beam forming unit 3 at multipliers (not shown) of each provided in beam formers 31 to 3M (M is an integer greater than one) for forming M fixed beams. The products are combined and then multiplied by weighting factors calculated in advance, and further combined, so that the phase and amplitude of the received signals are controlled, thereby forming a beam formed in a specific direction.
The M fixed beams are provided so as to cover, as uniformly as possible, a predetermined space region such as a sector. A beam power detection unit 5 measures power levels of the beams from the beam formers 31 to 3M at beam power detecting parts 51 to 5M, and notifies a beam output selection combining unit 6 of both the measured power levels and beam numbers thereof. The beam output selection combining unit 6 selects and combines one or more beams having large power levels by referring to the measured power levels, and then outputs the combined beam as received data.
With the above-described conventional multibeam system, the beam power detection unit 5 measures the power levels of all the fixed beams, and then a beam to be received is determined on the basis of the power levels. At this time, the resolution to an incoming direction of the received signal depends on the number of fixed beams.
Therefore, the resolution may be enhanced by increasing the number of fixed beams. This, however, leads to an inevitable increase in operation amount both of the beam formers 31 to 3M and of the beam power detection unit 5.